Some people don't realize that a grand jury is not an actual trial....it's the step BEFORE a trial. When a case is dropped by a grand jury and officers acquitted it is not just saying that they are not guilty but that we don't even need a real trial to determine that.
What makes these cases particularly egregious, sometimes in the face of video and other evidence is that the result is often pushed by prosecutors who work for exactly the same government system as the police. In fact, for most criminal cases, the police are part of the prosecution team. So essentially we have cops out raping and killing people, the public saying "Hey, wait this guy looks guilty from the video etc. we should have a trial to find out for sure." And the prosecutors defending their own colleagues by saying "Nah, we don't even need to have a real trial and hear evidence."
Can we forget magic blue costumes which allow rampant abuse for a moment and imagine what would happen if a citizen without a uniform performed these acts??? "Yes, this Down's Syndrome man was making noise in the theater where I'm an usher and I feared for my safety so I killed him." Or "A man was illegally selling loose cigarettes outside my building so I put him in a choke hold until he died." Or "I shot my neighbors dog because I have a gun and I can." What would happen to these civilians? Would they be charged and have a trial to determine guilt or innocence? We can't imagine a case where the evidence would not even be heard...other than for those with a magic badge.
That's not justice.